Sailing Magazine includes Knots 3D as one of five recommended knot guides for mobile offline use in their “How To” blog.
There is no shortage of knot-tying applications available for handheld devices and they can be a handy guide to remind you how to tie less well-known knots. Some are geared more towards fishing or climbing, but many applications include knots that sailors will use.
Knots 3D gives excellent descriptions of knots and their usage, including some history. There are no step-by-step instructions provided, but there are some notes on knot structure that can be helpeful. Animation shows the knot being tied very quickly and then you swipe your finger to see it step by step.